A mark imaging method in an ordinary exposure apparatus for manufacturing semiconductors will be described with reference to FIG. 7. Shown in FIG. 7 are a reticle R, a wafer W that is a substrate to be exposed, a projection optical system 1 in which the optical axis is the z axis, an optical system S for imaging marks that are to be observed, an illumination unit 2 for imaging the marks, a beam splitter 3, optical systems 4 and 5 for forming an image, an image sensing unit 6, an A/D converter 7, an integrating unit 8, an image processing unit 9, a stage driving unit 10, a stage 11 that is movable in three dimensions, and a position measuring unit 12, such as an interferometer, for measuring stage position.
Mark imaging in the exposure apparatus having the above-described structure is performed through the following procedure. First, the reticle R is moved by a reticle-stage moving unit (not shown) to a position at which a reticle mark RM can be observed. Similarly, the stage 11 is moved to a position at which it is possible to observe an observation mark WM on the wafer W. Initially, the wafer mark WM is illuminated by light flux, which is emitted by the illumination unit 2, via the beam splitter 3, reticle mark RM and projection optical system 1. Light flux reflected from the wafer mark WM reaches the beam splitter 3 again via the projection optical system 1 and reticle R. The light flux that has arrived from the projection optical system 1 is reflected by the beam splitter 3 and forms an image RM of the reticle mark and an image WM of the wafer mark on the image sensing surface of the image sensing unit 6 via the image forming optical system 5.
FIG. 2A illustrates an example of the observation marks whose images have been formed on the image sensing surface in the above description. The reticle mark RM and the wafer mark WM each comprise a plurality of identically shaped patterns. The reticle mark and the wafer mark have a mark pitch equivalent to a certain fundamental spatial frequency. The image sensing unit 6 converts the images of the marks formed on the image sensing surface into electrical signals (photoelectric conversion). The A/D converter 7 subsequently converts the output of the image sensing unit 6 to a two-dimensional digital signal sequence.
The integrating unit 8 in FIG. 7 executes integration processing along the direction Y of an area WP of the kind shown in FIG. 2A and converts the two-dimensional signal to a one-dimensional digital sequence S0(x), as shown in FIG. 2B. On the basis of the digital signal sequence S0(x) obtained by the conversion, the image processing unit 9 uses means, such as pattern matching or calculation of a center of gravity to measure the center positions of the reticle and wafer marks and to measure their relative positions.
The above-described method of detecting a mark position is one that is extremely useful in a position detecting apparatus that requires accurate detection of the mark position. In the above example of the prior art, however, the signal is a discrete sequence owing to the A/D conversion for the purpose of processing the electrical signal and, hence, the detected mark positions also are discrete values. The influence of this can no longer be neglected as the need for greater position detection accuracy grows.
To deal with this, the specifications of Japanese Patent Application Laid-Open Nos. 3-282715 and 10-284406 disclose techniques for converting discrete sequence signals to signals in a spatial frequency domain by an orthogonal transform and measurement mark position based upon the phase of a mark-specific spatial frequency component, thereby making it possible to detect position from discrete sequence signals in a highly accurate fashion.
However, in a case wherein the art disclosed in the specifications of Japanese Patent Application Laid-Open Nos. 3-282715 and 10-284406 is utilized, position must be measured mark by mark in order to measure the positions of both the reticle mark RM and wafer mark WM. In other words, a processing window must be set separately at the two areas of reticle mark RM and wafer mark WM, and the orthogonal transform, which places a burden on processing, must be performed twice. The problem that arises is a decline in throughput.